/**
 * 权重
 */
module.exports = function (tasks, weightTotal, context) {
    
    if (!tasks.length) return null;
    
    let max = tasks[0].weight, maxIndex = 0, sum;
    
    for (let i = 0; i < tasks.length; i++) {
        sum = (tasks[i].weight || 0) + Math.random() * weightTotal;
        if (sum >= max) {
            max      = sum;
            maxIndex = i;
        }
    }
    
    context.weightIndex += 1;
    context.weightIndex %= (weightTotal + 1);
    
    return tasks[maxIndex];
};